I'm not certain, but I think the answer is no. The HR20 uses esata drives, not usb ones.
At present, you can't transfer recorded programs from one HR20 to another and I don't foresee ever being able to transfer from an E* box to a D* box. As much as the networks rerun everything, I suspect you could get most of your programs back by just rerecording them when you go to D*. Count on more reruns than ever now that the writer's strike is here.But no way to ever txfr to D*'s HD receiver or ext hd?